IMO Kaspersky is the best the anti-virus program out there with the fastest updates for new security threats, but I have no expierence with their internet security suite since it is a new product to their line up. It could be worth giving it a try with the reputation Kaspersky has with security.
Also for Kaspersky anti-virus speed the first scan takes forever, but after that the scans just get faster and faster. The program is somehow able to detect if a file has been modified and will only scan it if there is a change.